1. Field of the Invention
The present invention relates to a hermetically sealed package container which employs a tungsten-copper composite having a high-level thermal conductivity and thermal expansion coefficient similar to those of semiconductor devices such as GaAs-FET (gallium arsenide field effect transistor), and GaAs MMIC (monolithic microwave integrated circuit) which are applied to low noise high frequency amplifying devices operating in the order of a GHz frequency band, for thereby preventing the aforementioned devices from being influenced by external environmental factors such as electromagnetic fields and moistures, and to a fabrication method thereof.
2. Description of the Prior Art
In recent years, along with rapid developments in wireless communication technology ranging from mobile telephone communication to artificial satellite communication, there has been an increasing demand for high frequency devices which can operate in frequency bands of single digit GHz to double digit GHz.
A GaAs compound semiconductor having a higher electron mobility in comparison to an Si semiconductor, can be directly attached onto a heat sink substrate due to the electronically nonconductive characteristic of the substrate itself. However, a disadvantage with the GaAs compound semiconductor is its low rupture strength. In order for the GaAs semiconductor to be employed as a microwave device, container packaging design must be carried out under a concept different from that of the Si semiconductor. Such a package structure is provided with a space for safely accommodating a semiconductor device in a desired portion therein, a circuit formed therein, and an air-tight lid for hermetically sealing the package.
A ceramic package has been widely employed for high power and high frequency devices. However, where alumina is used as an insulating ceramic, multi-layer package is necessary in order to fabricate a circuit, a sintering temperature for shaping thereof is high, and of which surface is plated with conductive metals material, as disclosed in U.S. Pat. Nos. 3,936,864/4,150,393/4,649,416.
Also, due to a frequency oscillation effect being induced resulting from a ring-shaped metallic layer provided in an upper portion of a ceramic container which is used to seal the container's lid, a valid frequency band tends to be disadvantageously narrow.
Although a metallic container may easily overcome such a disadvantage, input/output terminals are still difficult to introduce through the container because they must be insulated from and remain perpendicular to the walls thereof. Further, the container must be machined as disclosed in U.S. Pat. No. 4,672,151, thereby reducing productivity and increasing cost.
The microwave device provided in the container emits heat when power is supplied thereto. For an optimal operation of the device, the container substrate that receives the device thereon should be made of material with a sufficient thermal conductivity, for efficiently dissipating away the heat from the device and preventing the device from falling in the container. Additionally, thermal expansion coefficient of the container substrate should be similar to that of a semiconductor substrate to cyclic prevent stress build-up due to heating.
A tungsten-copper composite material is known to have similar characteristics to a GaAs FET, and a GaAs MMIC for microwave devices, as disclosed in U.S. Pat. Nos. 4,942,076/4,988,386/5,563,101. This is because the tungsten-copper composite as a package material provides good thermal conductivity due to evenly dispersed copper matrix having a high thermal conductivity through a tungsten skeleton that has a low thermal conductivity, and at the same time the thermal expansion coefficient of tungsten-copper composite is similar to that of GaAs.